17.10.13 Permitting tail calls

True if it is ok to do sibling call optimization for the specified
call expression exp. decl will be the called function,
or NULL if this is an indirect call.

It is not uncommon for limitations of calling conventions to prevent
tail calls to functions outside the current unit of translation, or
during PIC compilation. The hook is used to enforce these restrictions,
as the sibcall md pattern can not fail, or fall over to a
“normal” call. The criteria for successful sibling call optimization
may vary greatly between different architectures.

— Target Hook: void TARGET_EXTRA_LIVE_ON_ENTRY (bitmap *regs)

Add any hard registers to regs that are live on entry to the
function. This hook only needs to be defined to provide registers that
cannot be found by examination of FUNCTION_ARG_REGNO_P, the callee saved
registers, STATIC_CHAIN_INCOMING_REGNUM, STATIC_CHAIN_REGNUM,
TARGET_STRUCT_VALUE_RTX, FRAME_POINTER_REGNUM, EH_USES,
FRAME_POINTER_REGNUM, ARG_POINTER_REGNUM, and the PIC_OFFSET_TABLE_REGNUM.